I agree with JennyWren, except would take it further by saying "My working days are Mon.Tue., Thurs., Fri." or whatever they are, rather than emphasising the non-working day.
I would make sure you have a separate phone and e-mail address for work, and not turn them on / check them on the days you aren't being paid for.
I do agree with most that 4 days is not ideal, in that people do tend to expect you to do a full week.
Without knowing your job though, it's difficult to know if you can stick to fixed hours. Even if you can't, then can you think about the "average" hours you worked in FT, and then limit yourself to working 4/5 of that? that's what I did as a teacher working 0.6, when I'd done 36 hour in a week, I'd stop, working on the principal I'd done the 3/5 I was being paid for, but if you weren't strict with yourself, then you could go on forever, as it's the sort of job there's no end to.
I think it's good to have a positive way of feeding back to your managers / whoever is 'giving you the work'. "Thanks for allocating me this, it looks really interesting. Now, you know I'm paid for 4/5 of a week, do you want me to pick this up once I've completed X,Y,Z, or do you want me to start this now and put the others on hold?" type conversations. Just being very clear that they will obviously only get 4/5 of the work from you as they are now only paying out 4/5 of the money.
Again, depending on your work, would it help if they used the 1/5 of your salary to employ someone for a few hours a week in some kind of administrative or assistant role ? You could go in with a positive suggestion of how the company could be more productive that way, again, avoiding the concept that you need help.